Playback foreground error

So, I'm working on a project as an intern who uses Bugsnag to catch bugs in his web application. There is one error I am trying to fix, I cannot reproduce. This only affects 2 people out of all of our customers using the product. I did not write this code, I am only trying to fix the error. An error TypeError·Uncaught TypeError: Cannot set property 'selected' of undefined

, and it is in this function on the second line.

function selectInstaller(installer) {
      installers[installer.id].selected = true;
      selectedInstaller[installer.id] = installer;
}

      

        
        
        
      

    

It is assumed that a list of people exists and the user can select a person to display events on the calendar for that particular person. After selecting a user, this function is called.

function handleSelectInstaller(event) {
      var $btn = $(this);
      var $li = $btn.closest('li');
      $li.toggleClass('active');
      var installerAttributes = $btn.closest('li').data();
      if($li.hasClass('active')) {
        selectInstaller(installerAttributes);
        fetchInstallerCalendar();
      } else {
        previousInstallerCount = Object.keys(selectedInstaller).length;
        unselectInstaller(installerAttributes);
        syncView();
      }
    }

      

        
        
        
      

    

As you can see, the handleSelectInstaller function calls the selectInstaller function, which then raises an error. As I said, this is a rare situation where an error occurs and I can't see how to reproduce the error. I thought that maybe something might be wrong with the database for this particular person, but my manager looked at it and he said it looked right. He also tried to log into a user account and try to reproduce it that way, and he gave him no problem (so he couldn't reproduce it on his computer). But Bugsnag still reports this when the user logs on to their computer. Any ideas on how I can reproduce this error so that I can fix this issue? Or could it be completely out of my control?

EDIT : Here's what gets called on page load. The latter function will automatically select the user who is logged in. Then, when the user (installer) selects or deselects their name, the handleSelectInstaller function is called. The structure shows that we are passing to the handleSelectInstaller function.

function init(options) {
      var options = options || {};
      baseUrl = serverpath + "v1.0/calendar_events";
      selector = options.selector || '#calendar';
      colors = {
        default: '#59595C',
        disabled: '#ff9f89'
      };
      dateFormat = 'YYYY-MM-DD';
      type = $(selector).data('type');
      installers = {};
      installerArray = [];
      selectedInstaller = {};
      cache = {};
      cacheCalendar = {};
      currentMonth = {start: null, end: null}
      standardInstallerObject = {jobs: {}, daysOfWeekDisabled: {}, time_off: {}, color:null}
      previousInstallerCount = 0;
      setup();
    }

function setup() {
       setupListeners();
       setupDatePickers();
       $('.installer-name').text('Select Installer');
       syncEventTypes();
       syncJobStatuses();
       fetchInstallers(setupCalendar);
     }

function fetchInstallers(callback) {
      $.ajax({
        url: serverpath + 'v1.0/users?role=installers',
        type: "GET",
        dataType: 'json'
      }).done(function(response) {
        loadInstallers(response);
        if(typeof callback === 'function') callback();
      })
      .fail(function(xhr) {
        handleError(xhr);
        $('#calendar-loading-indicator').fadeOut();
      })
      .always(function() { });
    }

    function loadInstallers(response) {
      for(var i in response) {

        var installer = response[i];
        var id = installer.id;
        //need to extend / copy object since the object would be stored by reference not by value
        var copiedObject = $.extend(true, {}, standardInstallerObject);
        var copiedObjectCalendar = $.extend(true, {}, standardInstallerObject);
        cache[id] = copiedObject;
        cacheCalendar[id] = copiedObjectCalendar;
        if(installers[id]) continue;
        installers[id] = installer;
        installerArray.push(installers[id]);
      }

      if(type == 'installer') {
        var installer = installers[$.User.id()];
        selectInstaller(installer);
        $('.installer-pick-list li[data-id="'+ $.User.id() +'"]').addClass('active');
      }
    }